1. Field of the Invention
The present invention relates to an improved method for preparing certain oximes and oxime O-methyl ethers by reacting poorly water-soluble carbonyl compounds with salts of hydroxylamine or hydroxylamine O-methyl ether or the free base of hydroxylamine in the presence of certain phosphoric esters or salts thereof.
2. Description of Related Art
Methods for preparing oximes and oxime O-methyl ethers are known (see e.g. Houben Weyl, Methoden der Organischen Chemie, Vol. E14b, pp. 287-384, 1990, Patai Series, The Chemistry of Hydroxylamines, Oximes and Hydroxamic Acids, pp. 163-231, Wiley 2009). A known representative of the oxime substance class is cyclohexanone oxime, which is a precursor for preparing polyamide. Oximes and oxime ethers play an important role as intermediates in the manufacture of active ingredients of e.g. plant protection agents and pharmaceuticals.
Oximes and oxime O-methyl ethers are often prepared by reacting carbonyl compounds with hydroxylamine or hydroxylamine O-methyl ether. The hydroxylamine—if used as the free base—is commercially available, usually as a 50% aqueous solution. The free base is very reactive, which makes the handling technically difficult. In the reaction, particularly on an industrial scale, salts of hydroxylamine and hydroxylamine O-methyl ether are more easily handled and are, in addition, considerably more cost-effective. Known salts are hydroxylammonium sulphate and chloride and hydroxylamine O-methyl ether hydrochloride. In the industrial reaction, it is always advantageous to use convenient starting materials which are also easy to handle since they may be used, for example, as a solid or an aqueous solution.
In the reaction of carbonyl compounds with salts of hydroxylamine or hydroxylamine O-methyl ether or the free base of hydroxylamine, very polar solvents must be used due to the polar properties of hydroxylamine and salts thereof. Solvents used include water, alcohols or alcohol/water mixtures, pyridine or DMSO (see Houben Weyl, Methoden der Organischen Chemie, Vol. X/4, 1968, pp. 55-91, Chem. Ind. 240, 1969). Despite using such solvents, the salts of the particularly inexpensive hydroxylammonium sulphate are frequently not sufficiently reactive under these conditions, particularly when carbonyl compounds with very low water solubility are involved, for which reason the hydroxylammonium chloride is usually employed in the synthesis. After the reaction using the polar solvents, moreover, a complex reaction mixture usually results which, in addition to the desired product, comprises an equivalent salt, the strongly polar solvent and optionally water. Such reaction mixtures are generally unsuitable for direct further processing. The workup of such a reaction mixture is complex and expensive however, especially for industrial production, since the polar solvent must be completely removed, by distillation for example, before the aqueous salt solution can be separated off and disposed of. A procedure in water without organic solvent, but with addition of long-chain perfluorinated carboxylic acids as phase transfer catalysts, is described in the Russian Journal of Applied Chemistry Vol 75, 511, 2002. A disadvantage of this process is the use of expensive perfluorinated compounds and the limitation to carbonyl compounds which are liquid under the reaction conditions.
The disadvantages of the prior art have now been overcome by an improved method, wherein the reaction takes place in a mixture of at least 2 liquid phases (at least a biphasic system), and in which the desired oxime or the desired oxime O-methyl ether can be prepared with high chemical yield and high conversion rates. Especially with the method, all poorly water-soluble carbonyl compounds (particularly aldehydes, ketones and quinones) can in principle be favourably reacted with a hydroxylamine salt or a salt of hydroxylamine O-methyl ether.